column; 2, Mammae; 3, A placental embryo; 4, Four legs; 5, A single
well-developed toe in each foot provided with a hoof; 6, A bushy tail;
and 7, Callosities on the inner sides of both the fore and the hind
legs. The asses, again, form a distinct species, because, with the
same characters, as far as the fifth in the above list, all asses have
tufted tails, and have callosities only on the inner side of the
fore-legs. If animals were discovered having the general characters of
the horse, but sometimes with callosities only on the fore-legs, and
more or less tufted tails; or animals having the general characters of
the ass, but with more or less bushy tails, and sometimes with
callosities on both pairs of legs, besides being intermediate in other
respects--the two species would have to be merged into one. They could
no longer be regarded as morphologically distinct species, for they
would not be distinctly definable one from the other.

However bare and simple this definition of species may appear to be, we
confidently appeal to all practical naturalists, whether zoologists,
botanists, or palaeontologists, to say if, in the vast majority of
cases, they know, or mean to affirm anything more of the group of
animals or plants they so denominate than what has just been stated.
Even the most decided advocates of the received doctrines respecting
species admit this.

"I apprehend," says Professor Owen*, "that few naturalists nowadays, in
describing and proposing a name for what they call 'a new species,' use
that term to signify what was meant by it twenty or thirty years ago;
that is, an originally distinct creation, maintaining its primitive
distinction by obstructive generative peculiarities. The proposer of
the new species now intends to state no more than he actually knows;
as, for example, that the differences on which he founds the specific